1. Field of the Invention
The invention relates to a device for measuring a gas flow, of the type having a measurement chamber and an ultrasonic transceiver unit which can be attached to the measurement chamber over the openings thereof and which is provided with transmit and receive heads which are oriented against the openings of the measurement chamber, with membranes being arranged between the measurement chamber and the transmit and receive heads which are permeable to ultrasound waves, but largely impermeable to moisture and bacteria.
2. Description of the Prior Art
German PS4 222 286 describes an ultrasonic flow meter of the above type in which the transmitter and the receiver are arranged at a distance from each other along a measuring tube. The measuring length extends obliquely to the axis of a tubular measurement chamber through which the medium flows whose flow rate is to be determined. This flow meter is known as a spirometer for determining the lung capacity of the patient. In order to maintain hygiene, a sterile insertion tube is inserted into the measurement chamber with each new patient. The sterile tube is provided with measurement windows which are fitted so that they are situated over the openings. Membranes which are permeable to ultrasound signals but impermeable to moisture and bacteria are arranged in the measurement windows, so that the ultrasound signals along the measuring tube can pass through the sterile insertion tube. It is therefore unnecessary for the hospital personnel to autoclave the flow meter after every use, which is advantageous since the ultrasonic transceiver unit, in particular, are sensitive parts in the flow meter. In connection with the known ultrasonic flow meer, the membranes, which are arranged at a distance from the ultrasonic transmit and receive heads, can be foam rubber, in one example, and a Mylar.RTM. film, in another. In connection with the first example, in order to be able to reach the transmit or receive unit, the ultrasound signals must first pass through the relatively thick foam rubber membrane, and then a relatively large air gap. This transition from a relatively thick membrane to a relatively large air gap can lead to a high acoustical impedance, i.e. to a high sound wave reflection. This can lead to relatively large acoustical losses, so that an unacceptably low sound signal reaches the transmit or receive heads. A relatively low acoustical impedance is associated with the use of a Mylar.RTM. film, due to its extreme thinness, so that a receivable sound signal can reach the aforementioned heads. The disadvantage of Mylar.RTM. films which are attached as described is that they are so thin and sensitive that they cannot always withstand the mechanical stress they are exposed to when a pressure excess arises in the measurement chamber, which can cause the films to easily rip.